Galaxy Colosseum (Galaxy Arena in PAL regions) is a battle stage that appears in Mario Kart Wii during special online tournaments. It is essentially a round platform situated in outer space, specifically a Super Mario Galaxy/Rainbow Road-themed setting. Thus far, three tournaments have taken place on the arena.
In the tournament which took place between June 1st and June 10th, 2008, players had to compete for the fastest time by knocking a total of four giant Spiked Topmen off of the arena's edge using only Triple Mushrooms. Players could knock them away with a Mushroom boost.
Also, in the 2nd June tournament of the 15th to the 25th of 2009, it was the same, except the outer rings of the arena broke off, and players had to send six Spiky Topmen off of the stage.
For the 2nd tournament of March 2010, the player had to do the same as the 2nd June tournament of 2009 but with an exception: some Topmen could run away, and for this tournament, the player could use any Kart, Bike, or control scheme. Again, six is the required number to win.
According to the map of the stage, the size of this stage differs in each boss battle. This stage is the Mario Kart Wii version of the Water Arena from Mario Kart DS.